Last night I have received an automatic update to Plasma 5.10, libQt 5.9, etc; about 700 packages!
At the end of the update, Plasma5 has crashed immediately.
After restarting the computer (with "poweroff" from a VT), here is what happens:

The kernel starts properly.
The nVidia driver is working properly.
The graphic screen starts and the mouse cursor appears.
The screen remains black and nothing else happens.

From a Virtual Terminal I have tried "sddm" and "gdm"; same behavior.

Now I have managed to get into IceWM through "kdm".
Running whatever Plasma5 application from an IceWM console I get this message:
symbol lookup error: /usr/lib64/ undefined symbol: FT_Get_Font_Format
I have checked the libQt5 repository, but that library does not exist!
What can I do now?

I know that KDE developers have had problems with libQt >= 5.8; Plasma 5.10 beta was compiled against libQt 5.71.
Should I try a downgrade to libQt 5.71?